Pulsed oscillator



A. D. BLUMLEIN PULSED OSCILLATOR Feb. 7, 1950 2 Sheets-Sheet 2 Filed June 19, 1.946

I N V EN TOR. Ala/1 flower BlumZez'n, Deceawed flora): Vl/kez; Execazrzlx ATTORNEY:

Patented Feb. 7, 1 950 UNITED STAT S NT" OFFICE PULSED OSCILLATOR trix,'. Penzance,. Gornwall,

Industries Limited, Mid- Electric- & Musical:

England, assignor to dlesex, England, a. company of. Great. Britain Application June 19", 1946, Serial N 0. 677,673 lin- Great Britain January 27-, 1940 Section 1, Public Law 690,August 8, 19.46 Patent expires January 27, 1960 Claims; 1.

This. invention, relates to thermionic valve apparatus, for generating. high voltage impulses of short duration.

It is sometimes necessary to. operate thermionic valve apparatus for relatively short periods of time at relatively long, time intervals. Such a requirement arises, for example, in the case of transmitting. apparatus for radiating short bursts of radio frequency oscillations in the detection andlocation oi reflecting objects such as aircraft.

Intermittent operation. of this kind may conveniently be brought aboutby applying the requisite polarizingvoltagesto the. valves only during the time periods when. it isdesired that the apparatus. shall operate. Thus, the anode voltage maybe applied tothe-valves only during. the desired operative periods, and it is therefore desirable to have some convenient means for generating the required anode voltage. Moreover, as itis found thatin such circumstancesvalves are not damaged by the application of polarizing voltages greatly exceeding those, which are permissible for continuous operation, it is advantageous to generate extremely high voltages during the operative-periods.

It is an object. of the presentfinvention to provide apparatus adaptedtm generate high voltage impulses-of shortdurationz.

It is a further object of the present invention toprovide: apparatus for generating short bursts of: radio frequency oscillations in which residual oscillations aresuppressed;

According toone feature of. the present invention there is provided apparatus for applying to a load. highvoltage impulses of short duration at timeintervalslong in relationto said short duration, said apparatus comprising. a thermionic valve (or valves) arranged so as normally to be non-conductive, an? inductance connected in the output circuit of saidvalve and means for periodically causing said valve to conduct for a: predetermined time period substantially shorter than said time intervals: soas to buildup a current in said inductance and for thereafter causing said valve to return to the non-conducting state so as tosetf up said'high voltage'impulse across said inductance, and means for feeding said impulsesto saidload;

Preferably;. the. valve. is: rendered conducting and: non-conducting in. the required manner by applying between aacontrol' electrode. of' said valve and its cathode suitable biassing pulses.

Said inductance may: constitute thewhole or a part of the seriesinductive elements of a. time delay network terminated at1one-end= by an im pedance; preferably low in value, such that in operation reflection occurs; and terminated at its other end in a substantially reflectionless manner by said'load. 1

Said valve may conveniently be provided with a screening grid in Which case a series resistance and/or a resonant circuit may be connected between said screeninggrid and the source of positive potential for polarizing said grid so as to limit the anode current of said valve during the preliminary portion of said predetermined time periods but to cause the potential of said screening grid to rise rapidly to amaximum towards the end of said predetermined time periods.

If desired, said biassing voltage may be of sawtooth waveform, and additional biassing means may be provided for applying additional negative bias to the control electrode of said valve when said high voltage impulse is generated so as. to maintain said valve non-conducting.

Said, apparatus may conveniently be used to provide. polarizing voltages in apparatus for generating. short bursts of radio frequency oscillations.

Now we have found that when apparatus of the kind referredto above isoperatedthe. generated oscillations do not cease immediately at the end of. the. high voltage polarizing impulse,.sincev the oscillations set up-in. the oscillatory circuits of the apparatus persist for a time governed by the decrement of thesecircuits. It will be appreciated that the persistence of the oscillations may be undesirable ifv it is desired. to detect reflecting objects at close range, since the reflected. signal will return. in. a. very short time and may be masked by the residual. oscillations of the transmitter.

According to another feature of the present. in.- vention, therefore, there is provided apparatus for generating short bursts ofradio frequency oscillations including a valve and apparatus incorporating any or all.- of the above-mentioned features for applying a. high voltage impulse-betweenthe anode. and cathode. of said valve so-as to cause the generation of saidoscillations in whichlmeans are provided for applying a positive voltage impulse,- after atime delay of thesame orderas the duration of said high voltage impulse between the gridand cathode ofsaid valveof said oscillator wherebysaid oscillations are-more rapidly damped after the cessation of: said.- im pulse by grid current damping.

Saidmeans may conveniently comprise an in:- ductive potentiometer connected in: shunt with said inductance; an. intermediate tapping. point 2,496,980 a I I i on i zpc n ipmetergbeing connected; irdesired f a standing. negative; bi which is applied t the, I f I tnroughatime delaylnetwoi'hitc thecontrol sic control electrodes of. the valves 53 via? the choke, i I *trcdecisaidvalive oisaidoscillator.i= i z II I I 'l'fifro'm" externallsourcejof'bias (not shown),- j i In orderth at theinventionmaybemore fullyf f maybeineutralizedtobring'saidicontrolelec I trodes to a suitable operatingpotential. when I i I II I i i i i f i f I f I understood it will, now ,be described in greater I detail with reierenceltoth'e' drawings inwhich i I I then-assuag impulseis developed: across the a Figure lis,a circuit diagrampi apulsegen inductance B. I I I r I I 1 I -:--'=:=erator. ccnstructedaccording to cne embodimentif Byisuitahly chcosing the tappingsconnecting 2 :0. miIiQftghe=inventifln I I. theinductance8:to'theelectrcdesofthevalvesZ] i a? can bejarranged that the bulktoi thc ener y I I i I I Figure '2 is a detail of E a E the switching pulses to the circuit; of. Figure l, i i i :or: the inductance fie; absrirbed by? the valves, 3

I -Figure 3iisgaicircuitdiagramoi a pulse gen-' Iduringapproximatelyahalfcyclezoithe nature. I I 'i'"' eratorconstructedaccording toanotherembo'di-fg period of; the inductance Band its'ass'ociated'i I I t of the inventiomand. II I I straycapacitieslThusgwhen the pulsezdurae i Figure eisja circuitdiagramthat illustrates' gi tion, {isarranged to be 0.5- microsecond, .thein -g ductancje 18 should tune with its associatedi ca m B ;fi i fi' y l 'I 1 In order to: prevent theinductance 8 from I a he initial voltage-surgeadamn T I i I i I i I I I, a: modfi atiori'ofthe apparatus; showniniffigfi I I Refs nggnow to Figure 1 it will be, seen} that a I a pair or pentodevalves 3. is 'mployed in a push I i pull radio irequeirc y :ampliiien: I The control. f i 5 electr cci's of the; valve 33 are connected; to a 1 tunedc cuift t which; is arranged; to be supplied ivalve 155mm? I f with radio frequency energy em, lacoill 5 i The I I I a I I I' i I t or: the induct n ee or ,e'valvest-areEconnectcd toa p; s; the inductance of which is: ecu i I I rent. in the I cutputc I I I f pledito' a coil, i Ewhichf I erve i to apply the outinductanceii reverses. I '1 1 I I I I put of the generator to, for exam 1 an aeria I f If desired, an additional valve IB may be pro- I 1 T j i It. will b assumed that theic' rran'gement d j through a switch fiifia; to; the: I I

I I ftob e arrangedtoigenerateipurstsof I I I I I 'o f thevalveifs:toassist inrendeh? I Iradi' res dency} oscillations; the app ximatef I ggthe valvef i3;non; -conducting iat the-required. I I I I durationfoflthe-burstsbeingO5 microsec rid a'nd time. Thevalvei lii isnormally nan-ceme ting; 5, the:surstmeeurnngeveryscomicroseecnda4a.: butas scones-thevalveiais rendcred non-con;

, Fcr this'purpcse,;mens arejprovidedrcraa ductingibyftheswitching;lpuiseithevalve lfi'is I I I plying suitable polarizing voltages o the elec rendered conducting-by thehigh voltageimpulse enerated across inductance 8. due to the cone trodeseither/aware s olmakeithenropfi I I I I I I I I I I I nectionfoiits' contr l electrcdeito thelower f n ii i I I i d erative ito iced: said? radio oscillations to I said 7 aeuacniyIdu mg: th time {intervals when; itiis crtheniceetaneer3; endithus provides a negative frfififf .desired.that said oscillations hail be transmittedL I pulse at'its ancde 'whicliis applied tothe :con-

i f Byreascnofjthe factzthat,thevalvessarezthus; itrolelectrpdeorvalveil3lso,ensuringthat'the only operated intermittently, very much higher; I jv'alvle 13' :is 1 held: in the non-conducting; state. i I I polarizing voltages may be applied than could 40 Further, should the inductance 8' tend to oscil- I be used for continuous operation of such valves. late for more than a half period of oscillation, These high polarizing voltages are provided by the valve It will be rendered non-conducting and means of the inductance 8 connected between will cause the valve l3 to conduct again owing the anode of a valve l3 and its source of anode to the rise of voltage at the anode of valve I6.

voltage (1000 volts) indicated by the arrow 9 I The damping thus applied by the valve l3 will and shunted by the bypass condenser H0. The then suppress the oscillations in the inductance 8. valve 13 is normally non-conducting, but means In order to improve the efiiciency of the opera- (not shown), such as a multivibrator, are protion of the valve I3 it is preferred to apply a vided for applying a positive biassing voltage sawtooth shaped switching signal to its control pulse to the control electrode of the valve i3 an electrode. If the valve 53 is rendered fully conso as to render said valve conducting for a preducting at the beginning of the 4 microsecond determined time period and thereafter to cause period, the anode potential will fall suddenly to said valve suddenly to cease conducting so as a low value causing the screening electrode of to set up a high voltage impulse, which may have the valve to take an undesirably high current.

If, however, a sawtooth shaped switching pulse a peak voltage of 7000 volts; across said inductis applied to the valve l3 it can be arranged that ance 8. Said biassing pulse may conveniently have a duration of 4 microseconds and may be the screen grid current is maintained small. A arranged to terminate just before each 0.5 microcircuit for modifying a rectangular switching second interval duringwhich oscillations are to pulse of 4 microseconds duration to a suitable be radiated. sawtooth form is shown in Figure 2. The posi- The high voltage impulse set up across the tive rectangular pulse is applied through a reinductance 8 is applied to the valves 3 So as to sistance I! to a condenser 18 arranged in series cause them to become operative. The upper end with a diode valve 19. Shunted across the diode of the inductance 8 is connected to the center valve I9 is a leak resistance 20. When the posipoint of the tuning inductance of the tuned cirtive pulse is applied the diode I9 is rendered concuit 6 and thence to the anodes of the valves 3, ducting, short circuiting the resistance 20 and so that the high voltage impulse is applied bepermitting the condenser It to charge negatively tween the anode and cathode of each of the valves through resistance I7. When the positive pulse 3. A suitable tapping on the inductance 8 is ceases the diode I9 is rendered non-conducting connected to the screen grids of the valves 3 so and the negative charge in the condenser l8 that an appropriate potential is applied to these leaks away slowly through the leak resistance 20, electrodes simultaneously with the application restoring the potential of the condenser to its of the anode voltage. Further, the center tap original value, namely, earth potential, prior to of the tuning inductance of the tuned circuit 4 the arrival of the next positive pulse. The is connected to a still lower tapping on the incharge across the condenser I8 during the presductance 8 via the blocking condenser. 12, so that ence of a rectangular pulse is thus of substantially sawtooth waveform and is applied through a condenser 21 and leak resistance 22 to the grid of the valve H.

The switching pulses applied to the valves I3 or to the circuit of Figure 2 may be produced in any well known manner and may be shaped, if required, by suitable limiter stages. It is important that the valve l3 be rendered non-conducting as rapidly as possible so as to generate a high voltage across inductance 8 and to limit the power dissipated at the anode of the valve IS.

The circuit shown in Figure 1 enables very high voltages to be applied to the valves 3 far in excess of their normal rating, since the period of application of the voltages is so short that the defects that usually arise when valves are heavily overloaded are not so likely to occur. Similar remarks apply to the valve I3. Since the high voltages are of short duration the insulation of the inductance 7 will also not present serious difficulties.

Referring now to Figure 3, there is shown an alternative arrangement in which the inductance across which the high voltage impulse is developed forms part of a time delay network, Elements in this drawing corresponding to elements in Figure 1 of the drawings are given the same reference numbers. In this arrangement, the anodes of the four parallel connected valves I3 receive their polarizing voltage via the choke 23 and are coupled to the inductance 8 via the blocking condenser 25 and the inductance 26a, the purpose of which will be hereinafter explained. If desired, the choke 23 and condenser 2i may be omitted as illustrated in Fig. 4 and the 11-0. potentials in the cathode of valves l3 suitably adjusted by separate biassing means. It will be seen that a number of capacities 25 are connected between tapping points in the inductance 8 and the earthed end of the inductance 3 remote from the anodes of the valves l3, and the arrangement is such that these capacities 25 con stitute with inductance 8 a time delay network.

Let it be assumed that it is desired to apply to the anodes of the valves 3 a high voltage pulse of 2 microseconds duration at intervals of 1500 microseconds, and that the valves 3 are to draw 3 amperes at 6000 volts in each 2 microsecond period. The effective load resistance will thus be 2000 ohms. In this case, the time delay network constituted by the inductance 8 and the capacities .25 should be designed to have a characteristic impedance substantially equal to 2000 ohms and a time delay substantially equal to l microsecond.

The arrangement operates as follows. During time periods, which may conveniently be 20 microseconds, immediately preceding the 2 microsecond periods during which it is desired to generate the high voltage impulse, the valves is are caused to conduct and to build up current in the inductance 8 and thereafter are suddenly caused to become non-conducting. If it is assumed that the current built up in the inductance is 6 amperes, then the interruption of this current can be regarded as equivalent to the application of a current of 6 amperes in the reverse sense when the valves 43 are rendered non-conducting. The effective application of this reverse current will be in the proper sense to raise the anodes of the valves 3 to a high positive potential and thus to cause these valves to .presenta 2000 ohmload and thus to provide a matched -termination for the delay network. I-Ialf the applied current will thereiore'fiowthrough the load provided 'by th'e valves 3 and the other half of the current will propagate along the network, be reflected at the short circuited end in similar phase and return. The voltage wave will, however, be reflected in antiphase, so that after 2 microseconds the voltage across the valves 3 will collapse to zero and the whole of the applied current will continue to-iiow through the inductance 8, thus neutralizing the current built up in the inductance 6 during the 20 microsecond charging period and reducing the not current in the inductance 8 to zero. It will thus be seen that the network provides a high voltage pulse having a duration equal to twice the time delay of said network.

It will be appreciated that it is important that the load shall be of a value suitable to terminate the delay network in a substantially reflectionless manner, since otherwise reflections will occur and :further voltage pulses will be applied to the load at 2 microsecond intervals, which might be disadvantageous, particularly if as shown in the drawing the valves 3 are arranged to operate as a self-maintained oscillator, as oscillations may recommence when-these reflected impulses arrive. The load impedance may conveniently be adjusted by alteration of the coupling between the tuned circuit Band the aerial coupling coil 1.

Although it is preferred to short circuit one end of the delay network, any terminating impedance, which in operation will give rise to refiection, may be used. In such cases, however, subsidiary voltage pulses may occur at intervals after the main pulse, and precautions, such as the provision of grid current damping, may be necessary to prevent recommencement of oscillations.

The voltage impulse generated by the above arrangement .is substantially rectangular, which is particularly advantageous in the case of pulse transmitters for detecting the presence of reflecting objects since, as has previously been explained, the sharp fall of potential causes the oscillations to be suppressed rapidly and this'enables reflecting objects at ClOSe range to be detected. It is found, however, that the sharpness of the voltage impulse waveform sometimes reduced by the capacities of the valves and the stray capacities of "the circuits, etc, and it is therefore desirable to introduce inductances 26a, 26b and 260 as'shown to form with these capacities a .low pass filter of constant characteristic impedance and passing frequencies up to as high a frequency as possible. In general, the squareness of the pulse can be improved by giving the inductance 26a a value lying between CR and MQCR where C is the effective capacity of the anode circuit of the valves l3 and R is the characteristic impedance of the time delay network.

Similarly, the stray capacities 21 of the lead 28 through which the voltages are applied to the anodes of the valves 3 are built out by means of the inductance 261) and the efiect'ive capacity of the anode circuit of the valves 3 is built out by the inductance 260, so that the voltage generated in the time delay network may pass to the anodes of the valves 3 with only a small amount of distortion. In one practical case, the inductance 8 was split up into 20 sections and the condensers '25 were :given values of 25 niic'romicrofarads, the time delay network thus constituted having a characteristic impedance of 2000 ohms and the path between the network and the :load was effectively broken up into sections of 30 micromicrofarads "capacity by means such as the series inductance 26b.

In order to limit the power dissipated in the valves l3 which build up the current in the inductance 8, there may conveniently be included between their screening grids and the source of polarizing voltage for said grids a resistance 29 and a parallel resonant circuit comprising the inductance 30 and the shunt capacities 3|. This arrangement causes the potential of the screening grids to drop during the preliminary portion of the charging period, due to the voltage drop produced by the large screening grid current across the resistance 29, but if the resonant frequency of said circuit is suitably chosen it causes the potential on these electrodes to rise to a maximum towards the end of this period due to oscillation of the tuned circuit. In this way, the power dissipated in the valves l3 during the charging period is reduced without decreasing the current set up in the inductance 8 at the end of the charging period.

Although in the examples described above, the high voltage pulse has been applied to a load consisting of apparatus adapted to generate radio frequency oscillations upon the application of said pulse thereto, it will be understood that the load to which the pulse is applied may be of any desired form. Any number. of valves may be used in parallel for building up the current in the inductance, and the high voltage may, if desired, be taken from another inductance coupled to said first-mentioned inductance.

It will be understood that, in the case of apparatus for generating short bursts of radio frequency oscillations under the control of high voltage pulses developed by apparatus according to the invention, the valves providing the radio frequency oscillations may either be self-oscillating or may serve to amplify oscillations provided by a separate source of oscillations. In the latter case, it is only necessary to arrange that radio frequency oscillations are applied to said valves during the periods when said valves are operative. In the former case, it may be desirable to include between the inductance, across which the high voltage impulses are developed, and the anodes of said valves, a resistance shunted by a condenser and adapted to provide a negative bias for the anodes of said valves so as to prevent the generation of further oscillations should additional positive pulses be generated due to oscillation of said inductance with its parallel capacity.

The invention is particularly applicable for use with valves having oxide coated cathodes, since they are capable of supplying very large currents as compared with their normal rating. Where the invention is applied to valves having so-called bright emitters the invention does not in general allow for a very large increase in their power output although, of course, the advantage in using the inductance and the associated elements as a means for obtaining very large high tension voltages for short periods is still present.

Although the present invention is especially applicable when it is desired only to apply operating potentials to the electrodes of valves for relatively short periods during which they are required to be operative and during which they are heavily overloaded, the invention can, however, be applied generally. For example, it may be also applied to very large power valves in which case the very large voltagescontemplated in the present invention would not be regarded as being very large compared with the normal rated operating potential. V V

8 Whatis claimed is:

1. Apparatus for applying to a load, high voltage impulses of short duration at time intervals long in relation to said short duration, comprising at least one thermionic valve arranged so as normally to be non-conducting, an inductance connected in the output circuit of said valve, and means for causing said valve to conduct for a predetermined time period substantially shorter than said time intervals so as to build up a current in said inductance and for thereafter causing said valve suddenly to return to the non-conducting state so as to set up said high voltage impulse across said inductance, said inductance constituting the whole or part of the series inductance elements of a time delay network that is terminated at its end remote from said valve by an impedance, preferably low in value, such that in operation reflection occurs, and that is terminated at its other end in a substantially reflectionless manner by said load.

2. Apparatus according to claim 1 in which said means for causing said valve to conduct comprises a source of positive biassing pulses connected between the control electrode and cathode of said valve.

3. Apparatus according to claim 1 in which said biassing pulses are of sawtooth waveform.

l. Apparatus according to claim 1 in which said valve has an additional electrode connected to serve as a screening grid and in which there is included between said screening grid and a source of positive voltage a resistance whereby the voltage drop due to current flowing to said screening grid maintains the current in the output circuit of said valve at a low value during the preliminary portion of said predetermined time periods.

5. Apparatus for applying to a load, high voltage impulses of short duration at time intervals long in relation to said short duration, comprising at least one thermionic valve arranged so as to be non-conducting, said valve including an electrode connected to serve as a screening grid and an oscillatory circuit connected between said screening grid and a source of positive voltage whereby the voltage drop due to current flowing through said screening grid maintains a current in the output circuit of said valve at a low value during the preliminary portion of said predetermined time periods, said oscillatory circuit being so tunedthat in operation due to oscillations of saidcircuit the voltage of said screen grid rises to a maximum towards the end of said predetermined time periods, an inductance connected in an output circuit of said valve, means for causing said valve to conduct for a predetermin'ed time period substantially shorter than said time intervals so as to build up a current in said inductance and for thereafter causing said valve suddenly to return to the non-conducting state so as to set up said high voltage impulse across said inductance, and means for feeding said impulses to said load.

6. Apparatus for applying to a load, high voltage impulses of short duration at time intervals long in relation to said short duration, comprising at least one thermionic valve arranged so as to be non-conducting, said valve including an electrode connected to serve as a screening grid and an oscillatory circuit connected between said screening grid and a source of positive voltage whereby the voltage drop due to current flowing through said screening grid maintains a current in the output circuit of said valve at a low value during the preliminary portion of said predetermined time periods, said oscillatory circuit being so tuned that in operation due to oscillations of said circuit the voltage of said screen grid rises to a maximum towards the end of said predetermined time periods, an inductance connected in an output circuit of said valve, means for causing said valve to conduct for a predetermined time period substantially shorter than said time intervals so as to build up a current in said inductance and for thereafter causing said valve suddenly to return to the non-conductin state so as to set up said high voltage impulse across said inductance, said inductance constituting the whole or part of the series inductance elements of a time delay network that is terminated at its end remote from said valve by an impedance, preferably low in value, such that in operation reflection occurs, and that is terminated at its other end in a substantially reflectionless manner by said load.

'7. Apparatus for applying to a load, high voltage impulses of short duration of time intervals long in relation to said short duration, comprising at least one thermionic valve arranged so as normally to be non-conducting, an inductance connected in the output circuit of said valve, means for causing said valve to conduct for a predetermined time period substantially shorter than said time intervals so as to build up a current in said inductance and for thereafter causing said valve suddenly to return to the nonconducting state so as to set up said high voltage impulse across said inductance, said inductance constituting the whole or part of the series inductance elements of a time delay network that is terminated at its end remote from said valve by an impedance, preferably low in value, such that in operation reflection occurs, and that is terminated at its other end in a substantially reflectionless manner by said load, and means for feeding said impulses to said load, said means for causing said valve to conduct comprising a source of positive biassing pulses connected between the control electrode and cathode of said valve, said last means being so connected to said inductance that when in operation said high voltage impulse is generated, and means for applying an additional negative bias to the control grid of said valve so as to maintain said valve non-conducting.

8. Apparatus for applying to a load, high voltage impulses of short duration at time intervals long in relation to said short duration, comprising at least one thermionic valve arranged so as normally to be non-conducting, an inductance connected in the output circuit of said valve, means for causing said valve to conduct for a predetermined time period substantially shorter than said time intervals so as to build up a current in -.said inductance and for thereafter causing said valve suddenly to return to the non-conducting state so as to set up said high voltage impulse across said inductance, and means for feeding said impulses to said load, said valve having an additional electrode connected to serve as a screening grid, said apparatus including between said screening grid and a source of positive voltage a resistance whereby the voltage drop due to current flowing to said screening grid maintains the current in the output circuit of said valve at a low value during'the preliminary portion of said predetermined time periods.

9. Apparatus for applying to a load, high voltage impulses of short duration at time intervals long in relation to said short duration, comprising at least one thermionic valve arranged so as normally to be non-conducting, an inductance connected in the output circuit of said valve, means for causing said valve to conduct for a predetermined time period substantially shorter than said time intervals so as to build up a current in said inductance and for thereafter causing said valve suddenly to return to the non-conducting state so as to set up said high voltage impulse across said inductance, and means for feeding said impulses to said load, said load comprising a generator for generating short bursts of radio frequency oscillations which generator includes a valve that is normally inoperative, said means for feeding said impulses to said load comprising means for applying said impulses between the anode and cathode of said generator valve so as to cause said generator valve to become operative to generate said oscillations, and wherein at least one inductance is connected between the anode of said generator valve and said first inductance so as to constitute with the stray capacities of the circuit a filter adapted to pass said impulses without substantial distortion of its waveform.

10. Apparatus for applying to a load, high voltage impulses of short duration at time intervals long in relation to said short duration, comprising at least one thermionic valve arranged so as normally to be non-conducting, an inductance connected in the output circuit of said valve, means for causing said valve to conduct for a predetermined time period substantially shorter than said time intervals so as to build up a current in said inductance and for thereafter causing said valve suddenly to return to the non-conducting state so as to set up said high voltage impulse across said inductance, and means for feeding said impulses to said load, said load comprising a generator for generating short bursts of radio frequency osciilations which generator includes a valve that is normally inoperative, said means for feeding said impulses to said load comprising means for applying said impulses between the anode and cathode of said generator valve so as to cause said generator valve to become operative to generate said oscillations, and wherein means are provided for applying a positive voltage'impulse, after a time delay of the same order as the duration of each of said high voltage impulses, between the grid and cathode of said generator valve whereby said oscillations are more rapidly damped after the cessation of each of said impulse by grid current damping.

DOREEN WALKER, Emecutria: of the Last Will and Testament of Alan Dower Blumlein, Deceased.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,695,042 Fearing Dec. 11, 1928 1,848,133 Kroger Mar. 8, 1932 2,394,389 Lord Feb. 5, 1946 2,408,824 Varela Oct. 8, 1946 2,412,710 Bradley Dec. 17, 1946 

